Inrush limiting circuits are commonly used in conjunction with DC-DC converters. DC-DC converters usually incorporate large value capacitors in parallel with their input terminals for filtering purposes. If input voltage is suddenly applied to the power supply, inrush currents flow from the power source to charge the input capacitance. Inrush current limiters are circuits that control the rise time of the applied input voltage, thereby limiting the inrush current that flow upon application of power to tolerable levels.
In many applications, it is also desirable or mandated that no output should be produced by the power supply until and unless the input voltage is sufficiently high. Since DC-DC converters are regulating devices, for a constant output load, input current increases inversely proportional to input voltage. Therefore, by inhibiting production of output power from the power supply until the input voltage is above a sufficient level, the input current magnitude at low input voltages is controlled. This often prevents a damaging condition.
DC/DC converters are electronic devices that use switching components, such as field effect transistors (FETs) to transform voltage from one level to another. Typically, the output voltage is regulated and protected against short circuits. In many cases, the input and output potentials are galvanically isolated from each other.
In an FET, current flows along a semiconductor path called the channel. At one end of the channel, there is a source electrode, and at the other end, a drain electrode. The physical diameter of the channel is fixed, but its effective electrical diameter is changed by applying voltage to a gate electrode. The conductivity of the FET depends, at any given time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in current from the source to the drain. In this way, the FET switches current on or off.
Typically, FETs used for power switching are enhancement mode types, that is, they are normally non-conducting. When a gate voltage above a certain threshold is applied, the FET becomes conducting. Such FETs are used to control current flow and are available in two gate polarities; N channel and P channel.
Among many applications, inrush limiters are used in spacecraft, satellites and in high energy physics instrumentation where they are subjected to many forms of radiation damage. When electrical components are exposed to radiation, they behave differently. For example, when an N channel FET is exposed to relatively low radiation levels, the gate threshold voltage ultimately falls close to zero. In this condition, the FET conducts current with little or no applied gate voltage. In other words, the FET is uncontrollable because the current running through the channel cannot be shut off.
Inrush limiters designed for general purpose use are typically constructed with N channel FETs because, for any given die size transistor, the N channel FET has a lower on resistance than a correspondingly sized P channel FET.
To use electrical components in high radiation environments, they are radiation-hardened to withstand the damage caused by radiation. The radiation hardening process usually involves removing or adding some specific element or ions to the materials used for making the components. Being radiation hardened, the gate threshold voltage experiences minimal change after exposure to radiation. One method for chemically radiation hardening DC/DC converters is disclosed in U.S. Pat. No. 3,836,836 to Cowett, Jr. (Cowett).
Radiation-hardened components, however, have limited sources, are expensive and take a long time to produce, creating higher prices and longer delivery times for the radiation tolerant circuits that incorporate the hardened materials. It is desirable, therefore, to provide electrical components with ordinary (non-hardened) materials that can function when exposed to radiation.